Method and apparatus for encoding optical power and non-payload data in an optical signal

ABSTRACT

A method and apparatus for encoding optical power and non-payload data in an optical signal is described. The method involves producing a dither modulating signal having an amplitude of indicative of the optical power in the optical signal and having a phase representing the non-payload data, and modulating the optical signal with the dither modulating signal. The apparatus involves a waveform generator for producing an amplitude adjusted waveform having an amplitude responsive to the optical power of the optical signal and a binary phase shift keying modulator for binary phase shift keying the amplitude adjusted waveform in response to the non-payload data to produce a dither modulating signal having an amplitude indicative of the optical power in the optical signal and having a phase representing the non-payload data.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 09/473,714,filed Dec. 29, 1999, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to signal encoding, and more particularlyto a method and apparatus for encoding and decoding optical power andnon-payload data in an optical signal.

BACKGROUND OF THE INVENTION

In modern communications networks, communications traffic is oftencarried on optical fibers. A plurality of transmitters may be connectedto a first end of a fiber through an optical multiplexer to transmit acorresponding plurality of optical payload signals thereon. Eachtransmitter includes a semiconductor laser whose output opticalintensity is modulated between a first predefined intensity representinga logical “0” and a second predefined intensity representing a logical“1” to generate an optical payload signal. Using a method calledwavelength division multiplexing (WDM), the lasers transmitting on thefiber are chosen such that each laser generates a signal having a uniqueoptical wavelength, permitting a plurality of payload signals to beoptically multiplexed onto a single optical fiber without becomingmixed. An optical demultiplexer is connected to an opposite end of thefiber to demultiplex the plurality of optical payload signals onto acorresponding plurality of optical links for receipt by a correspondingplurality of receivers.

To manage such a communications network efficiently, it is necessary tomonitor power of the optical signal, to ensure that the signal will beproperly propagated through the optical fiber to be detected at areceiver. One method of facilitating optical power measurement isdisclosed in U.S. Pat. No. 5,513,029 to Roberts, wherein an opticalpayload data signal is submodulated with a dither signal encoded with apseudo random code, and having a submodulation depth which is maintainedconstant relative to mean optical power of the submodulated opticalpayload data signal. A pseudo random code is used to broaden theemission spectrum of lasers used in optical transmitters to reducenon-linear optical effects such as Stimulated Brillouin Scattering(SBS).

In addition, to effectively manage an optical system it is desirable totransmit communications and control information between nodes on anoptical network. This is typically done by including communications andcontrol information, also referred to as overhead data, in the payloaddata signal or by submodulating the payload signal with overhead data.

Thus, submodulation of an optical signal has been used for either powermonitoring or for transmission of communications and control data.Typically, where submodulation is used for power monitoring,communications and control data is transmitted in payload data and wheresubmodulation is used for payload data, power monitoring is notprovided.

Generally, it is not possible to simply add the communications andcontrol data to the pseudo random codes used for power monitoring asthis can result in a submodulation depth greater than an allowablelimit. Exceeding such a limit may limit the distance the light cantravel in the fiber, requiring more optical amplifiers at shorterspacings.

Furthermore, while channel power information is directly measurable whenthe channels are separate, prior to WDM multiplexing or afterdemultiplexing, it is difficult to directly measure the power ofindividual channels of a WDM signal. One existing method of determiningchannel power of a WDM signal involves optically demultiplexing the WDMsignal to retrieve individual optical payload signals, converting theindividual optical payload signals into individual electrical signalsand then measuring the power of each such electrical signal. However,this method requires the use of a relatively expensive opticaldemultiplexer and may not, therefore, be economical.

Thus, there is a need for a way to transmit power information andnon-payload data in an optical signal without excessive depth ofmodulation and without interfering with the payload data, whilefacilitating economical power measurement.

SUMMARY OF THE INVENTION

The present invention addresses the above need by providing a method andapparatus for encoding optical power and non-payload data in an opticalsignal, which involve producing a dither modulating signal having anamplitude indicative of the optical power in the optical signal andhaving a phase representing non-payload data, and modulating the opticalsignal with the dither modulating signal.

Such a system facilitates the use of a constant depth of modulation ofthe optical signal while enabling both optical power and non-payloadinformation to be transmitted, independently of the payload data encodedin the optical signal. Thus, the optical signal need not be demodulatedto enable the payload data to be combined with non-payload data, therebyreducing the expense of the optical system.

In one embodiment, a depth of modulation is produced in the opticalsignal in response to the amplitude of the dither modulating signal andthe amplitude of the dither modulating signal may be varied in responseto the optical power of the optical signal, to maintain a constant depthof modulation.

The optical mean power of the optical signal may be measured and arepresentation of a measured depth of modulation of the optical signalmay be produced by squaring a representation of the optical signal toproduce a tone signal having a tone signal amplitude representing themeasured depth of modulation. A digital representation of the tonesignal amplitude and a digital representation of the optical mean powermay be used by a processor to compute a ratio of modulation depth tomean optical power, and this ratio may be used for controlling awaveform generator to adjust the amplitude of a reference waveform toproduce an amplitude adjusted waveform. This amplitude adjusted waveformmay be binary phase shift keyed (BPSK) by the non-payload data toproduce the dither modulating signal. The light output of a laser may becontrolled in response to the dither modulating signal to modulate theoptical signal. Thus, instead of encoding the dither signal with apseudo random code as in the prior art, the dither signal isphase-encoded by non-payload data. The amplitude of the dither signalencodes optical power information while the phase of the dither signalencodes non-payload data. Power and non-payload data are thus encoded inthe same dither signal.

In accordance with another aspect of the invention, there is provided anoptical signal produced by the method described above.

In accordance with another aspect of the invention, there is provided anapparatus for encoding optical power and non-payload data in an opticalsignal. The apparatus includes provisions for producing a dithermodulating signal having an amplitude indicative of the optical power inthe optical signal and having a phase representing the non-payload dataand provisions for modulating the optical signal with the dithermodulating signal.

In accordance with another aspect of the invention, there is provided anapparatus for encoding optical power and non-payload data in an opticalsignal. The apparatus includes a waveform generator for producing anamplitude adjusted waveform having an amplitude responsive to theoptical power of the optical signal and further includes a binary phaseshift keying modulator for binary phase shift keying the amplitudeadjusted waveform in response to the non-payload data to produce adither modulating signal having an amplitude indicative of the opticalpower in the optical signal and having a phase representing thenon-payload data.

Preferably, the waveform generator includes a tone generator forgenerating a tone signal representing a depth of modulation in theoptical signal due to amplitude of the dither modulating signal. Thewaveform generator may include an optical power signal generator forgenerating an optical power signal representative of optical mean powerof the optical signal.

The apparatus may further include a reference waveform generator forgenerating a reference waveform of constant amplitude and may include again controlled amplifier for amplifying the reference waveform toproduce the amplitude adjusted waveform. The gain controlled amplifiermay be controlled by a processor circuit which produces a gain controlvalue for controlling the gain controlled amplifier in response to theoptical mean power and depth of modulation of the dither modulatingsignal.

Preferably, the processor circuit is programmed to adjust the gaincontrol value such that the dither modulating signal has a constantmodulation depth in the optical signal. The processor circuit is alsopreferably programmed to compute a ratio of the modulation depth tooptical mean power and to adjust the gain control value to maintain theratio of modulation depth to optical mean power constant.

The apparatus may further include a laser having a bias current controlfor receiving the dither modulating signal to modulate an optical signalproduced by the laser in response to the dither modulating signal.

In accordance with another aspect of the invention, there is provided amethod of extracting non-payload data from an optical signal modulatedwith a dither modulating signal carrying the non-payload data byproducing an electrical representation of the optical signal,demodulating the electrical representation to extract a binary phaseshift keyed signal, and demodulating the binary phase shift keyed signalto obtain the non-payload data.

The method may further include squaring the binary phase shift keyedsignal to produce a tone signal having an amplitude representative ofthe power of the optical signal. The value representing the amplitude ofthe tone signal may be multiplied by a predefined value to determine thepower of the optical signal.

In accordance with a further aspect of the invention, there is providedan apparatus for extracting non-payload data from an optical signalmodulated with a dither modulating signal carrying the non-payload data.The apparatus may include an optical to electrical signal converter forconverting the optical signal into an electrical signal, a firstdemodulator for demodulating the electrical signal to extract a binaryphase shift keyed (BPSK) signal from the electrical signal and a BPSKdemodulator for demodulating the binary phase shift keyed signal toobtain the non-payload data.

In accordance with another aspect of the invention, there is provided amethod and apparatus for measuring optical power in individual opticalsignals of a composite optical signal. An optical to electrical signalconverter converts the composite optical signal into a compositeelectrical signal, a demodulator demodulates the composite electricalsignal to produce a composite dither signal comprised of a plurality ofdither modulating signals and a squarer squares the individual dithermodulating signals to produce a composite squared signal including aplurality of tone signals representing optical power in respectiveindividual optical signals.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,

FIG. 1 is a block diagram of a wavelength division multiplexed networkaccording to one embodiment of the invention;

FIG. 2 is a block diagram of a first transmitter shown in FIG. 1;

FIG. 3 is a block diagram of a non-payload data modulator of the firsttransmitter shown in FIG. 2;

FIG. 4 is a block diagram of a first receiver shown in FIG. 1.

DETAILED DESCRIPTION

As shown in FIG. 1, a wavelength division multiplexed (WDM) network isshown generally at 10. The network 10 includes a plurality oftransmitting stations, or optical transmitters 12 connected to anoptical medium 20 by an optical multiplexer 16. The optical medium 20 isconnected to an optical demultiplexer 22 which is further connected to aplurality of optical receivers 24. An optical amplifier such as thatshown at 200 or a plurality such optical amplifiers maybe positioned atlocations along the optical medium to amplify optical signals on theoptical medium 20 to compensate for optical attenuation on the medium20. Effectively, the network functions to permit optical signalsproduced by the optical transmitters 12 to be received at respectiveoptical receivers 24.

Referring to FIG. 2, a first transmitter is shown generally at 14. Thefirst transmitter 14 is representative of each of the opticaltransmitters 12 shown in FIG. 1.

The first transmitter 14 includes an optical source 26 which in thisembodiment includes a semiconductor laser. The optical source 26 iscontrolled by a bias current controller 28 for adjusting the intensityof light produced by the optical source 26. Such light is provided to anexternal modulator 30 which receives a payload data modulating signalfrom an encoder 32 which receives a conventional high frequency payloaddata signal. This signal may provide data to the encoder at a rate ofapproximately 2.5 Gigabits per second, for example. Such data mayoriginate at a telecommunications switch or router, for example, and maybe provided to the encoder 32 by a trunk line, for example.Alternatively, the optical source 26 may be modulated directly by thepayload data modulating signal.

In this embodiment, the payload data modulating signal produced by theencoder 32 directs the external modulator 30 to modulate the lightproduced by the optical source 26 to produce a high frequency opticalsignal which is provided by the external modulator 30 to an opticalmedium 33 and to an optical tap 34. The optical medium 33 is incommunication with the optical multiplexer 16 shown in FIG. 1 to providean optical signal thereto at a wavelength λ, for multiplexing onto theoptical medium 20.

Referring back to FIG. 2, the optical tap 34 taps off or couplesapproximately 3% of the optical signal on the optical medium 33 toproduce an optical tap signal which is provided to a non payload datamodulator 36. The non payload data modulator 36 receives a digital bitstream of non payload data at a much lower frequency than the payloaddata provided to the encoder 32. Such a digital bit stream may originateat a control or distributed control system for controlling devices inthe optical system or for providing for communication between devices inthe optical system, for example. This type of digital bit stream mayhave a frequency of several hundred kilobits per second, for example.

The non payload data modulator 36 acts as an apparatus for encodingoptical power and non-payload data in an optical signal. In general, thenon-payload data modulator produces a dither modulating signal having anamplitude indicative of power in the optical signal produced on theoptical medium 33 and having a phase representing the non payload data.The dither modulating signal is used to control the bias controller 28to modulate the optical signal produced by the optical source 26 beforeit is modulated by the external modulator 30. Alternatively, the dithermodulating signal may be used to control the external controller to sumthe dither modulating signal with the payload data modulating signalproduced by the encoder 32.

Referring to FIG. 3, the non payload data modulator 36 is illustratedand includes a waveform generator 38 for producing at an output 39thereof an amplitude adjusted waveform having an amplitude A₀ responsiveto power of the optical signal. The non payload data modulator furtherincludes a binary phase shift keying (BPSK) modulator 40 for binaryphase shift keying the amplitude adjusted waveform in response to thenon payload data to produce the dither modulating signal, at an output41 thereof such that the dither modulating signal has an amplitudeindicative of power in the optical signal and a phase representing thenon payload data.

Effectively, the dither modulating signal has the form:

-   -   g(t)=A₀ cos(ω₁t+Φ), while the non-payload data has the binary        value “1”; and    -   g(t)=−A₀ cos(ω₁t+Φ), while the non-payload data has the binary        value “0”.        where A₀ is the amplitude of the amplitude adjusted waveform and        ω is the frequency of a reference waveform.

To produce the value A₀, the waveform generator 38 includes a opticalpower signal generator 42 for generating an optical power signalrepresentative of optical mean power of the optical signal and includesa tone generator 44 for generating a tone signal representing a currentdepth of modulation in the optical signal due to the dither modulationsignal.

The optical power signal generator 42 includes an optical to electricalconverter 46, which in this embodiment includes a PIN diode whichconverts the tapped optical signal to a photocurrent. The optical toelectrical converter 46 may further include a transimpedance amplifier50 which amplifies and converts the photocurrent to a voltage. Thevoltage produced by the transimpedance amplifier 50 is provided to afirst low pass filter 52 and then amplified by a first amplifier 54 tobring the signal produced by the low pass filter 52 up to a levelcompatible with an analog to digital converter 56 of the optical powersignal generator 42. The analog to digital converter 56 produces a timevarying digital representation of the signal produced by the firstamplifier 54, the average amplitude of the digital representation beingrepresentative of optical mean power of the optical signal tapped offfrom the optical medium 33 shown in FIG. 2.

Still referring to FIG. 3, the tone generator 44 includes a signalsquaring circuit 58 which squares the signal produced by the firstamplifier 54. The squaring circuit 58 has the effect of squaring thedither modulating signal carried by the optical signal on the opticalmedium 33, to produce a waveform represented by the following equation:${g^{2}(t)} = {\frac{A_{o}^{2}}{2}\left( {1 + {\cos\left( {{2\varpi\quad t} + {2\phi}} \right)}} \right)}$

Thus, a tone signal having a frequency of 2ω and an amplitude of$\frac{A_{o}^{2}}{2}$is produced by the squaring operation. The signal produced by thesquaring circuit 58 is provided to a second low pass filter 60 whichremoves any high frequency noise produced by the squaring operation. Thesignal produced by the second low pass filter is provided to a secondamplifier 62 for scaling the signal for compatibility with a secondanalog to digital converter 64 which produces a time varying digitalrepresentation of the amplified tone signal. The amplitude of thedigital representation thus represents the amplitude of the tone signal.

The digital representation of the optical mean power signal produced bythe first analog to digital converter 56 and the digital representationof the tone signal produced by the second analog to digital converter 64are provided to a processor circuit shown generally at 66. In thisembodiment the processor circuit 66 includes a digital signal processor68 and a microprocessor 70.

The digital signal processor 68 includes a reference waveform generator72 for generating a reference waveform of constant amplitude. Thisreference waveform is provided to a gain controlled amplifier 74 foramplifying the reference waveform to produce an amplitude adjustedwaveform, having an amplitude A₀. The digital signal processor 68 andthe microprocessor 70 cooperate to produce a gain control value forcontrolling the gain controlled amplifier 74 in response to the opticalmean power signal and the depth of modulation of the optical signal dueto the dither modulating signal as represented by the tone signal.

Effectively, the processor circuit 66 produces the reference waveformand the gain control value which are supplied to the gain controlledamplifier 74, which produces the amplitude adjusted waveform having anamplitude responsive to power of the optical signal, for controlling thebinary phase shift keying modulator 40.

To produce the gain control value G, the digital signal processor 68performs a discrete fourier transform operation on the digitalrepresentation of the optical mean power signal and the digitalrepresentation of the tone signal to produce an optical mean signalpower value representing optical mean power and to produce a tone signalamplitude value T, representing amplitude of the tone signal at thefrequency 2ω. These two values are passed to the microprocessor 70 whichis programmed to compute a modulation depth d from the tone signalamplitude value T, according to the formula:d={square root}{square root over (2T)}In order to produce the gain control value G, preferably, the processorcircuit 66 is programmed to compute a ratio of the modulation depth d,to optical mean power p and to adjust the gain control value G tomaintain this ratio constant.

In particular, in this embodiment, the gain control factor is producedby first calculating an error value, e, according to the followingrelation: $e = {\frac{\alpha\quad d}{p} - r}$where

-   -   α=constant scale factor    -   d=modulation depth of modulating dither signal    -   p=optical mean power value    -   r=desired ratio of modulation depth to optical mean power    -   e=error value

The constant scale factor α is determined empirically by measurements ofoptical calibration signals and is influenced by attenuation and gainand by optical to electrical transfer functions of the optical toelectrical converter 46, the first low pass filter 52, the firstamplifier 54, the squaring circuit 58, the second low pass filter 60,the amplifier 62 and the second analog to digital converter 64. Thedesired ratio r is a constant identifying the percentage of themodulation to be applied to the optical signal to represent themodulating dither signal and in this embodiment the ratio r=0.1represents 10% modulation.

After determining the error value e, the gain control value G iscalculated according to the following relation:G=G ₀(1−e)where G₀ is an initial gain scaling factor, e is the error valuecalculated in the equation above and G is the gain value to be appliedto the amplifier 74 shown in FIG. 3.

From the foregoing, it will be appreciated that as the entity$\alpha\frac{d}{p}$tends to the desired modulation ratio r, the error e becomes zero, inwhich case the gain presented to the amplifier is simply G₀. As theentity $\alpha\frac{d}{p}$increases, the error increases and the gain G decreases, therebydecreasing the amplitude A₀ of the dither modulating signal to the BPSKmodulator 40 shown in FIG. 3. Conversely, as the entity$\alpha\frac{d}{p}$decreases, the error value e tends towards −r and the gain value G tendstowards G₀₍1+r), which in this embodiment is 1.1 G₀. The amplitudeadjusted waveform produced by the waveform generator 38 is thusdependent upon the entity $\alpha\frac{d}{p}$which is defined by the non payload data signal and the optical meanpower signal. Hence, the amplitude adjusted waveform has an amplituderesponsive to power of the optical signal and this waveform is used tocontrol the amplitude of the dither modulating signal while thenon-payload data is used to control the phase of the dither modulatingsignal produced at the output 41 of the BPSK modulator 40.

Referring back to FIG. 1, each transmitter is configured produce to asimilar dither modulating signal but at a different frequency ω fromeach other transmitter. Therefore, each of the transmitters 12 generatesa respective dither modulating signal, having a respective uniquecarrier frequency c each of which is orthogonal to each other to permitdetection and separation.

The optical signal produced on the optical medium 33 is effectively acomposite optical signal as it is comprised of the payload signal andthe dither modulating signal, hence it may be referred to as a compositeoptical signal. Since each transmitter produces a separate compositeoptical signal on its own “channel” or different wavelength of light,the composite optical signals produced by the transmitters arehenceforth referred to as channel composite optical signals.

The channel composite optical signal from the first transmitter 14 issupplied to a corresponding input of the optical multiplexer 16 forWavelength Division Multiplexing (WDM) with other channel compositeoptical signals produced by respective other optical transmitters. Thus,the optical multiplexer produces a WDM composite optical signalcomprising the channel composite optical signals from each opticaltransmitter, on the optical medium 20.

The optical demultiplexer 22 receives the WDM composite optical signaland splits it into respective channel composite optical signals. Eachchannel composite optical signal is supplied to a respective opticalreceiver 24.

Referring to FIG. 4, a first optical receiver is shown generally at 25.The first receiver 25 is representative of each of the receivers 24shown in FIG. 1. The first receiver acts to extract payload data fromthe received channel composite optical signal and includes an apparatus81 for extracting non-payload data from the channel composite signalcarrying payload data, after conversion from optical form to electricalform. To do this, the first optical receiver 25 includes an inputterminal 71 for receiving a channel composite optical signal. The inputterminal 71 is connected to an input 73 of an optical-to-electrical(O-E) converter 75, such as a PIN diode and transimpedance amplifier asdescribed above, for example, to provide the channel composite opticalsignal thereto. The O-E converter 75 is configured to produce a channelcomposite electrical signal at an output 76, the channel compositeelectrical signal being proportional to the channel composite opticalsignal appearing at the input 73. The channel composite electricalsignal is supplied to a highpass filter 79 to remove low frequencycomponents therefrom. The output of the highpass filter 79 providespayload data from the received channel composite optical signal.

The channel composite electrical signal is also supplied to theapparatus 81 for extracting non-payload data. Effectively, the apparatusfor extracting non-payload data includes a first demodulator showngenerally at 83 for demodulating the channel composite electrical signalto extract a binary phase shift keyed (BPSK) signal therefrom andincludes a BPSK demodulator 88 for demodulating the BPSK signal toobtain the non-payload data. The first demodulator 83 is provided by alowpass filter 78 having an input 77 for receiving the channel compositeelectrical signal and having an output 80 for providing the binary phaseshift keyed signal therefrom. The output 80 is connected to an input 84of the BPSK demodulator 88. In this embodiment, the BPSK demodulator isconfigured to detect 180 degrees phase shifts in the BPSK signal appliedto the input 84 and to produce, at an output 89, a received digitalnon-payload data signal wherein binary values represent phase shifts inthe BPSK signal. The output 89 of the BPSK demodulator thereforeprovides the non-payload data and may be connected to a microprocessor90, for example, to provide the non-payload data thereto for use inresponding to control or communication information encoded in thenon-payload data.

Referring back to FIG. 1, the network 10 may further include a channelpower monitoring unit 96 connected to the optical medium 20 to monitorchannel power of individual composite optical signals making up the WDMcomposite optical signal. In this embodiment, the channel powermonitoring unit 96 includes an optical interface and an apparatus 100for measuring channel power of a composite optical signal. The opticalinterface includes an optical tap 102 at a point 104 on the opticalmedium 20 between the optical multiplexer 16 and the opticaldemultiplexer 22, so as to tap off a portion of the WDM compositeoptical signal carried on the optical medium 20 to produce a tapped WDMcomposite optical signal. As described above, the optical tap may divertapproximately 3% of the optical signal power carried by the opticalmedium 20 for use as an optical tap signal.

The optical tap 102 is in communication with an optical-to-electrical(O-E) converter 106, such as a PIN diode and transimpedance amplifier asdescribed above, to produce an electrical representation of the tappedWDM composite optical signal. The optical to electrical conversionprocess destroys the wavelength separation between the individualchannel composite optical signals in the tapped WDM composite opticalsignal, destroying the multiplexed nature thereof. The optical toelectrical conversion process does, however, produce an electricaltapped WDM signal which is, in effect, representative of a superpositionof all of the channel composite optical signals.

The output of the O-E converter 106 is connected to an amplifier 107 foramplifying the electrical tapped WDM signal for receipt by the apparatus100 for measuring channel power. The apparatus 100 for measuring channelpower includes a demodulator for demodulating the amplified electricaltapped WDM signal to remove payload components of the individual channelcomposite electrical signals, thereby producing a composite lowfrequency signal. In this embodiment, the demodulator is provided by alowpass filter 108 which removes high frequency signals corresponding tothe payload signals and passes low frequency signals in the frequencyrange of the dither modulating signals of respective transmitters.

The low frequency signal output by the lowpass filter 108 thusrepresents a superposition of the individual channel dither modulatingsignals produced at each transmitter respectively.

The apparatus 100 for measuring channel power further includes a squarer110 for simultaneously and individually squaring each of the individualdither modulating signals to produce respective tone signals whichtogether comprise a composite tone signal.

Each tone signal has the form: $\begin{matrix}{{T_{n}(t)} = {\left( \frac{A_{n}^{2}}{2} \right)\left( {1 + {\cos\left( {{2\omega_{n}t} + {2\phi}} \right)}} \right)}} & (3)\end{matrix}$where A_(n) is the amplitude of the n^(th) individual channel dithermodulating signal and ω_(n) is the frequency of the channel non-payloadsignal produced by the n^(th) transmitter.

Thus each squared representation of a respective channel dithermodulating signal is represented by a respective tone signal having afrequency at twice its original carrier frequency. As each of thechannel dither modulating signals making up the composite channelnon-payload signal is a BPSK encoded signal, the squarer 110 has theeffect of simultaneously removing any BPSK induced phase shifts from thechannel dither modulating signals.

The composite tone signal from the squarer 110 is applied to a secondlowpass filter 112 which removes high frequency noise created by thesquaring operation to produce a filtered composite tone signal. Adigital representation of this signal is then produced by an A/Dconverter 114. The digital representation is processed by a digitalsignal processor 116 using a fast Fourier transform, for example, todetermine the amplitudes of respective tone signals.

The digital signal processor 116 outputs digital values representing theamplitudes T₁ to T_(n) of respective tone signals to a microprocessor118 for calculation of individual channel powers. Alternatively, the DSPitself may calculate the individual channel powers. Knowing theamplitude of any given tone signal, the amplitude of the correspondingchannel dither modulating signal in the tapped WDM composite opticalsignal is given by:A _(n)(tapped)={square root}{square root over (2T _(n))}where T_(n) is the amplitude of the n^(th) tone signal.

The amplitude of the corresponding channel dither modulating signal inthe (non-tapped) WDM composite optical signal is given by:$A_{n} = \frac{A_{n}({tapped})}{\beta}$where β represents a scale factor influenced by signal loses and gainsdue to electronic components between the optical medium 20 and the A/Dconverter 114. The value β would be approximately 0.03, for example, ifsignal losses and gains balance to zero as this represents the tappingof 3% of the optical signal by the optical tap at point 104.

The power of the corresponding composite optical signal is given by:$P_{ko} = \frac{A_{n}}{r}$where r is the desired ratio of modulation depth to optical mean powerused by the corresponding transmitter. Where the modulation depth is setby the transmitter to be 10%, for example, the value r is set to 0.1.

Referring to FIG. 1, channel power information can be used by themicroprocessor 118 in a variety of ways. For example, if the monitoringunit 96 is configured to measure channel power at both an input and anoutput of the optical amplifier 200, the monitoring unit 96 cancalculate an optical gain of the amplifier 200 for each channel andcontrol the gain of the amplifier 200 by providing negative feedback soas to correct any spectral distortion caused by the amplifier 200. Themonitoring unit may be an integral part of the amplifier 200 or may be astandalone unit, for example.

As well, the monitoring unit can be used for network diagnostics. Forexample, if the amplitude of the tone signal produced for a particularchannel falls below a certain threshold, an alarm can be raised alertingmaintenance personnel of a potentially faulty optical transmission line,for example. Alternatively, if the tone signal is present at themonitoring unit, but the associated payload signal is not present at theappropriate receiver, the fault probably exists between the monitoringunit and the receiver.

While specific embodiments of the invention have been described andillustrated, such embodiments should be considered illustrative of theinvention only and not as limiting the invention as construed inaccordance with the accompanying claims.

1. A method of encoding optical power and non-payload data in an opticalsignal, the method comprising: a) producing a dither modulating signalhaving an amplitude indicative of said optical power in said opticalsignal and having a phase representing said non-payload data; and b)modulating said optical signal with said dither modulating signal. 2.The method as claimed in claim 1 wherein modulating said optical signalcomprises producing a depth of modulation in said optical signal, inresponse to said amplitude of said dither modulating signal.
 3. Themethod as claimed in claim 2 wherein producing a depth of modulationcomprises varying said amplitude of said dither modulating signal inresponse to said optical power of said optical signal to maintain aconstant depth of modulation.
 4. The method as claimed in claim 3wherein producing said depth of modulation comprises measuring opticalmean power in said optical signal.
 5. The method as claimed in claim 4wherein producing said depth of modulation comprises producing arepresentation of a measured depth of modulation of said optical signal.6-10. (Cancelled)
 11. An optical signal produced by the method ofclaim
 1. 12. An apparatus for encoding optical power and non-payloaddata in an optical signal, the apparatus comprising: a) means forproducing a dither modulating signal having an amplitude indicative ofsaid optical power in said optical signal and having a phaserepresenting said non-payload data; and b) means for modulating saidoptical signal with said dither modulating signal.
 13. An apparatus forencoding optical power and non-payload data in an optical signal, theapparatus comprising: a) a waveform generator for producing an amplitudeadjusted waveform having an amplitude responsive to said optical powerof said optical signal; and b) a binary phase shift keying modulator forbinary phase shift keying said amplitude adjusted waveform in responseto said non-payload data to produce a dither modulating signal having anamplitude indicative of said optical power in said optical signal andhaving a phase representing said non-payload data.
 14. The apparatus asclaimed in claim 13 wherein said waveform generator comprises a tonegenerator for generating a tone signal representing a depth ofmodulation in said optical signal due to amplitude of said dithermodulating signal.
 15. The apparatus as claimed in claim 14 wherein saidwaveform generator comprises an optical power signal generator forgenerating an optical power signal representative of optical mean powerof said optical signal.
 16. The apparatus as claimed in claim 15 furthercomprising a reference waveform generator for generating a referencewaveform of constant amplitude.
 17. The apparatus as claimed in claim 16further comprising a gain controlled amplifier for amplifying saidreference waveform to produce said amplitude adjusted waveform. 18-33.(Cancelled)